Add lists of numbers respecting a relative shift between the starts of the lists.
The shifts must be non-negative.
The list of relative shifts is one element shorter
than the list of summands.
Infinitely many summands are permitted,
provided that runs of zero shifts are all finite.

We could add the lists either with foldl or with foldr,
foldl would be straightforward, but more time consuming (quadratic time)
whereas foldr is not so obvious but needs only linear time.

(stars denote the coefficients,
frames denote what is contained in the interim results)
foldl sums this way: